Members constituting a transmission line tower can be roughly divided into angular steel members having an L-shaped cross-section and tubular steel pipes. Since steel pipes have a higher rigidity than angular steels, many steel pipes are used in a large transmission tower. Surfaces of steel members and surfaces of steel pipes are plated for corrosion protection treatment when a transmission tower is constructed. Therefore, high corrosion protection capability is achieved on the surfaces of steel members and steel pipes. In contrast, when many existing transmission towers were constructed, the technique for plating inner surfaces of steel pipes had not yet been established. Therefore, in existing transmission towers that are mainly constituted by steel pipes, plating on the inner surfaces of their steel pipes is uneven in many cases. For this reason, in those existing transmission towers constituted by steel pipe members, corrosion on the inner surfaces of the steel pipes (rusting on the inner surfaces) tends to readily occur due to the influence of the natural environment over time.
Once corrosion (rusting) occurs, this corrosion (rusting) progresses rapidly. Therefore, if corrosion on an inner surface of a steel pipe is found, the corrosion needs to be dealt with immediately. Conceivable methods for completely removing corrosion include partial replacement of a corroded steel pipe, and overall reconstruction of the transmission tower itself. However, partial replacement of a steel pipe requires production of a jig for temporarily supporting a load and assembly of scaffolding, resulting in large-scale, long-term work, high costs, and long-term suspension of power transmission.
Overall reconstruction of a transmission tower itself requires not only removal of the existing transmission tower and construction of a new transmission tower, but also construction of a temporary transmission tower for supporting power transmission lines after removing the existing transmission tower until the construction of the new transmission tower is finished. Therefore, much more costs are required and a longer term of work is taken than in the partial replacement work for a steel pipe. Thus, the partial replacement of a steel pipe and the overall reconstruction of a transmission tower itself pose many problems.
As a method for preventing progression of corrosion in a transmission tower without requiring the partial replacement of a steel pipe and the overall reconstruction of the transmission tower itself, a method of filling the inside of a transmission tower (steel pipe) with synthetic resin has been disclosed (e.g., Patent Document 1). In Patent Document 1, an internal space of a transmission tower steel pipe is filled from the bottom to the top with synthetic resin by inserting an extruder into the internal space of the transmission tower steel pipe that constitutes an existing hollow steel pipe transmission tower, and gradually pulling up the extruder while extruding the synthetic resin (styrofoam) from the extruder.